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Uniform bounds for bilinear symbols with linear $K$-quasiconformally embedded singularity

Marco Fraccaroli, Olli Saari and Christoph Thiele

Vol. 18 (2025), No. 9, 2293–2323
Abstract

We prove bounds in the strict local L2(d) range for trilinear Fourier multiplier forms with a d-dimensional singular subspace. Given a fixed parameter K 1, we treat multipliers with nondegenerate singularity that are push-forwards by K-quasiconformal matrices of suitable symbols. As particular applications, our result recovers the uniform bounds for the one-dimensional bilinear Hilbert transforms in the strict local L2 range, and it implies the uniform bounds for two-dimensional bilinear Beurling transforms, which are new, in the same range.

Keywords
phase space localization, time-frequency analysis, modulation-invariant operators, uniform estimates
Mathematical Subject Classification
Primary: 42B15, 42C15
Milestones
Received: 19 February 2024
Accepted: 29 October 2024
Published: 5 September 2025
Authors
Marco Fraccaroli
Basque Center for Applied Mathematics
Bilbao
Spain
Olli Saari
Centre de Recerca Matemàtica
Bellaterra
Spain
Departament de Matemàtiques
Universitat Politècnica de Catalunya
Barcelona
Spain
Christoph Thiele
Mathematisches Institut
Universität Bonn
Bonn
Germany

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